Large-scale Model Tests Research Articles

Study on the stability of shield tunnel face in clay and clay-gravel stratum through large-scale physical model tests with transparent soil

The soil layers that are traversed by urban tunnel are usually complex. The failure mode of the tunnel face under complex soil conditions with different layers plays a key role for the construction and design of the tunnel. In order to investigate the response of the excavation face under different soil conditions and buried depth of the tunnels, two different types of transparent soils are used in the present paper to carry out six large-scale physical model tests by self-designed model device. Two high-precision motoring linear control platforms are utilized to simultaneously control the optical laser and a Charge Coupled Device (CCD) high speed camera aiming to perform “CT scan”. The failure model of the excavation face is obtained through a three-dimensional program created by the team of this research. Based on the physical tests, it can be observed that the displacement and support pressure curve of the excavation face with respect to the withdrawal of the rigid plate can be divided into two stages in the clay stratum and three stages in the clay-gravel composite stratum. By conducting image analysis on the pictures captured by the CCD high speed camera, the two-dimensional failure zone of tunnel face is obtained. In the clay stratum, the failure zone forms a funnel that reaches the surface for small buried depths. Nevertheless, for larger depths, it consists of a sliding triangle and an overlying loosen rectangle without reaching the surface. In the clay-gravel stratum, the failure zone consists of a triangle and a funnel for small depths, while for larger depths, it consists of a triangle and a rectangle. However, the three-dimensional reconstruction of the failure mode provides more detailed information about its spatial distribution, constituting a useful guideline for the design and construction of tunnels.

Dynamic Behavior of Double Steel Sheet Pile Cofferdam under Different Wave Actions

Double-row steel sheet piles, which have good dynamic stability, strong deformability, high seepage resistance, and strong engineering adaptability, have been widely used as coastal defense structures. The paper investigates the dynamic behavior of the double-row steel sheet pile cofferdam under the actions of breaking-wave and standing-wave through large-scale flume model test. High-frequency optical fiber technology was applied to obtain the stress data on the steel sheet pile. It was found that the standing wave produced a cycle action of crest pressure and trough suction to the DSSP cofferdam, forming a scour pit at the toe of the seaside steel sheet pile. The breaking wave did not produce obvious reflection and suction against the DSSP but incurred huge crushing energy and large deformation on the DSSP cofferdam. The bending moment on the steel sheet pile under near the breaking wave is about 2 to 3 times that under the standing wave. The deformation of the steel sheet pile may cause leakage of the joint and soil erosion of the in-filled sands, even resulting in failure of the DSSP cofferdam.

Method for calculating horizontal drain induced non-linear and large strain degree of consolidation

Consolidating dredged clay slurries using a combination of vacuum pressure and prefabricated horizontal drains (PHDs) is widely used in practice. This is a large strain problem, but there is no existing large strain theory for PHD induced consolidation. A method with explicit equations has been proposed to consider both mechanical and geometrical non-linearities in analyzing PHD induced consolidation. The method considers stepwise variation of properties using imaginary time concept. An imaginary time is determined by the condition of continuity of degree of consolidation before and after changing the properties. Then the method was applied to analyze a large scale model test of vacuum consolidation with PHD. Two analyses were conducted. One considered both the mechanical and geometrical non-linearity (large strain), and another only considered mechanical non-linearity (small strain). The results of large strain analysis agree with the measured settlement curve and excess pore pressures well. While the small strain analysis under-estimated the rate of consolidation significantly. The results from this study indicate that for analyzing PHD induced consolidation of clay slurries considering the effect of large strain is important, and the proposed method can be a useful design tool.

Investigation of Deformation Pattern and Movement Law of the Huge-Thick Conglomerate Stratum by a Large-Scale 3D Model Test with Distributed Optical Fiber Sensor Monitoring.

Mining activities under the circumstances of huge-thick stratum occurrence commonly result in dynamic response of the working face. It is crucial to understand the rock failure and movement of the huge-thick stratum in order to prevent dynamic hazards. This paper introduces distributed optical fiber sensor (DOFS) monitoring into a large-scale model test to investigate the deformation pattern and movement law of the huge-thick conglomerate (HTC); the monitoring results are verified by numerical simulation. The results indicate that DOFS monitoring captures the spatiotemporal evolution of zoning development in the overburden deformation. The deformation field of HTC is illustrated, and there exists a strain basin that can be used to estimate the movement law of HTC. The average strain variability Ex, a new homogenization index for characterizing the overburden deformation, is proposed to describe the broken rules of the HTC. The numerical simulation proves the feasibility of the DOFS monitoring method and the correctness of the deformation pattern and movement law. This study provides efficient methods for DOFS monitoring utilization to investigate mining engineering problems and could be beneficial for unearthing the mechanisms of deep ground rock deformation.

Study on the Geo-Stress Loading and Excavation Unloading Devices of the Large-Scale Photoelastic Model Test for Deep-Buried Tunnels

At present, theoretical analysis, numerical simulation, and other methods cannot be used to properly solve the problems associated with the stability and bearing capacity of the surrounding rock and its supporting system, the interaction between the supporting structure and surrounding rock, and the sharing role of each supporting structure, all of which commonly occur in deep tunnels. The model test method represented by the photoelastic test is still an important approach to study this kind of problem. In view of the deficiency of the current loading system of the photoelastic model test, we developed a geo-stress loading system for the photoelastic model test, which can simulate the in situ geo-stress environment of unidirectional loading, bidirectional equal pressure, bidirectional unequal pressure, and tridirectional unequal pressure. The universal retaining force loading rod can realize the stability and effective compensation of loading, which is an original design. According to the principle of umbrella-shaped expansion and contraction mechanism, an excavation unloading device for the photoelastic model test is developed, which can realize the simulation of various degrees of displacement release in the excavation process of deep tunnels and other underground projects. The loading simulation test and excavation unloading simulation test show that the geo-stress loading system and excavation unloading device developed in this paper are flexible, exhibit good performance, and can fully achieve their respective test functions. The combination of two devices can compensate for the insufficiency of the current photoelastic model test and will promote the application of photoelastic model tests in underground engineering applications such as deep tunnel projects.

A Large-Scale Model Test System for Stability Study of Tunnel Under Static-Dynamic Load

In order to study the stability of tunnels under static-dynamic load, a new large-scale static-dynamic model test system was developed. The system was composed of a static loading unit, a dynamic loading unit, a data acquisition unit and a test bench, which were specially designed to overcome the technical problems of large-scale static-dynamic model test, such as dynamic loading and control, high-rate strain measurement and easy operation. The system used a model of 3600 × 3600 × 2500 mm in size to study a larger load disturbance area than static tests. It can accurately apply three-dimensional gradient static stress and dynamic load with adjustable amplitude (or magnitude), frequency and waveform to simulate the true geo-stress field and disturbance field, and accurately and rapidly collect displacement, stress, strain, velocity, acceleration and other important parameters of the model. In addition, it was convenient to adjust the scale of the model, remove and retain the test model. The system was used to carry out a static load model test and a static-dynamic load model test with a tunnel as a prototype. The results of the tests were consistent with the numerical results by FLAC-3D, which proved the reliability and accuracy of the system.

Large-scale field model tests of landslide dam breaching

The breaching of landslide dams is a severe geological disaster, and the resulting breaching floods significantly threaten people in downstream areas. In this study, a large-scale physical model was constructed in a natural river channel using unconsolidated dam material with a wide grain size distribution. Terrestrial laser scanning was used to capture three-dimensional geometric information of the landslide dam during the breaching process. The results showed that the dominant erosion pattern during dam breaching was surface progressive erosion. The breaching process could be divided into three stages based on different erosion characteristics: headward erosion, rapid erosion, and attenuated erosion. Longitudinal and lateral evolution models were proposed based on the field test results. The longitudinal evolution model describes the erosion at the dam crest and the sedimentation at the dam toe during the breaching process by defining the erosion and rotation points. The lateral evolution model revealed different lateral erosion characteristics at different cross-sections. Both the longitudinal and lateral evolution models were validated using two dam break events of the Baige landslide. The evolution models proposed in this study provide a scientific reference for numerical simulations of dam breaching and the prevention and mitigation of landslide dams.

Evaluation of Micropiles With Different Configuration Settings for Landslide Stabilization Based on Large-Scale Experimental Testing

In this study, a large-scale model test was performed to investigate the effect of the single-row and double-row micropiles on the landside stabilization. For two different testing configuration settings, the bending moment along the micropiles, failure mode, and force condition were captured and compared. It is found that the landslide thrust on piles was distributed in a triangular shape. The piles in the front row carried greater pressure than the piles in the rear row. The resistance of the sliding body behind the pile was distributed in a parabolic shape, and mainly concentrated on the middle of the pile. The piles were destroyed due to the combined shearing and bending impact applied near the slipping surface. The boundary of the failure zone was from the position of two times the pile diameter under the slipping surface to the position of two and a half times the pile diameter above the slipping surface. Under the action of the landslide, each row of piles deformed at the same time. The capability of landslide stabilization for double-row piles was better than that of a single-row pile. The sections of the pile above slide surface were mainly subjected to negative bending moments and were distributed mainly within the pile length range of one-third of the anti-sliding section above the sliding surface. The pile body of the embedded section located in the range of ten times the pile diameter below the sliding surface was subjected to a positive bending moment.

Evaluation of the Water Shielding Performance of a Capillary Barrier System through a Small-Scale Model Test

Capillary barrier (CB) systems consisting of a fine-grained soil layer placed over a coarse-grained soil layer can generally provide a water-shielding effect, increasing the slope stability of soil structures during rainfall. In order to improve the water-shielding performance of CB systems, laboratory model tests have been previously conducted under various conditions; notably, large-scale model tests are especially required. The inefficiency in increasing the production time of CB models until now explains their high cost. In this paper, we propose a laboratory small-scale CB (SSCB) model test for a quick and efficient evaluation of the function of a CB system. In this model test, differently from previous studies, a side drainage flow in the direction of the inclined sand layer was set as the no-flow condition; moreover, the laboratory SSCB model tests were performed by considering three rainfall intensities (i.e., 20, 50, and 100 mm/h) under the lateral no-flow condition. The results showed that the larger the rainfall intensity, the shorter the diversion length was of the CB system. To evaluate the effectiveness of the SSCB model test proposed in this study, the diversion length was estimated by an empirical equation under the lateral flow condition based on hydraulic conductivity functions and the soil water characteristic curves of sand and gravel and then compared to the results of the SSCB model tests. It was hence demonstrated that the water-shielding performance of the CB system can be efficiently evaluated through SSCB model tests under the lateral no-flow condition, rather than through large-scale model tests.

Experimental Investigation of Load-Bearing Mechanism of Underwater Mined-Tunnel Lining

A series of model tests were performed to investigate the load-bearing mechanism of a mined railway tunnel lining under water pressure. To investigate the load-bearing characteristics of different types of linings, a fully closed water pressure exerting device for a noncircular section tunnel was invented. A large-scale model test (1:30) under combined water and soil pressures was conducted to investigate the mechanical characteristics, deformation, stress distribution, crack development process, and failure mode of the underwater mined-tunnel lining. The test results indicated that for the high-speed railway tunnel of Class IV surrounding rock with a design speed of 350 km/h, both the drainage lining and the waterproof lining were controlled by a small eccentric compression under the two test conditions. One had only water pressure, and the other had a variable water pressure and constant soil pressure. The key sections for controlling instability were the bottom of the wall and the inverted arch. The ultimate water head of the drainage lining was 49 m, and the ultimate water head of the waterproof lining was 78 m. In comparison with the drainage lining, the waterproof lining could significantly improve the water-pressure resistance. Thus, design loads of 30 and 60 m are recommended for the drainage and waterproof lining structures, respectively.

Strand element analysis method for interaction between cable and saddle in suspension bridges

A stable cable-saddle connection relying on the frictional resistance is essential for the safety of suspension bridges. However, the frictional resistance cannot be adequately quantified due to the unclear contact forces, giving rise to a thorny anti-slip problem that particularly troubles the design of multi-tower suspension bridges. In this paper, the cable-saddle interactions are therefore studied by means of theoretical modeling and experimental verification. First, the feasibility and strategy of analyzing the PPWS (prefabricated parallel-wire strand) cable from the perspective of strand are demonstrated. The geometric and mechanical models of 37-, 61-, 91- and 127-wire strands are created, and a strand element analysis (SEA) method with recursive algorithm is then developed. Large-scale model tests are further conducted to investigate the cable-saddle frictional mechanism and to verify the SEA method. Finally, a typical multi-tower suspension bridge is taken as an example for an extended case study. The results show that the proposed SEA method can offer reliable predictions for the contact behavior of multi-strand cables in the saddle. The strand tensions at the critical slip state are unevenly distributed, while slight changes are present on the average tensions of the strand on both sides of the saddle. The use of two vertical plates can improve the frictional resistance by about 30%, and adopting a fewer-wire strand specification and arranging the strands in a higher pattern is also beneficial to improve the cable-saddle frictional resistance.

Experimental study on the lateral distribution of the sidewall friction of large and deep-water caissons

The insufficiency of research on the calculation of the sidewall friction of large and deep-water caissons lies in that the longitudinal and lateral distribution of the sidewall friction is not clear. In this paper, a large-scale (1:70) model test was conducted based on the caisson foundation of the main pier at the Shanghai-Suzhou-Nantong Yangtze River Bridge to explore the longitudinal and lateral distribution of the sidewall friction during the dynamic and stable sinking process of caissons. The result shows that the sidewall friction along different longitudinal sections increases first and then decreases, with the peak value near the step. The lateral distribution of the sidewall friction is in the form of a parabola of “small in the middle and large at the ends.” On a cross section, the distance between the median point of friction and the midpoint of the section is about 0.3 times the side length. Also, this paper established a simplified four-segment model to calculate the sidewall friction of large and deep-water caissons.

Experimental and numerical modeling on the liquefaction potential and ground settlement of silt-interlayered stratified sands

Recent seismic events indicate that the simplified liquefaction-evaluation procedures are incapable of depicting general trends in liquefaction damage for stratified sands interlayered with silts. The conditions and mechanisms affecting the liquefaction potential of stratified sands exist in the field and ground settlement after liquefaction remain poorly understood. This work aims to investigate the seismic response of nonhomogeneous soil deposits by large-scale model tests and numerical simulations using an advanced constitutive model. A comprehensive experimental program was undertaken in which a total of three shake-table tests were performed on uniform sand and two stratified-sand deposits interlayered with different thicknesses of silt to investigate the ground settlement and distribution and dissipation of excess pore pressure during and after shaking. The shake-table test results and the numerical simulations of the silt-interlayered stratified sands, first indicate that the thickness of the silt seam has a significant influence on the liquefaction resistance of stratified-sand deposits beneath the silt layer. The second conclusion of this study reveals that the thickness and coefficient of consolidation of the silt and the liquefied sand below the silt layer significantly alter the degree of dissipation after the shake, and this causes different deformation/settlement at the ground surface. Therefore, there will be probably inaccuracies in applying simplified liquefaction evaluation procedures to the actual soil profile characterized by various patterns of layering in the field.

Large-scale model tests of a single pile and two-pile groups for an offshore platform in sand

Estimating pile group efficiency for open-ended steel piles in small group arrangements is a challenging task for designers. This paper reports on the large-scale experimental campaign performed for the BorWin gamma offshore converter platform, which involved single piles and two-pile group systems on a scale of 1:10. The experimental works included installation, dynamic end-of-driving tests, dynamic restrike tests, and static load tests of a single pile and a pair of two-pile groups in densely compacted, artificially prepared homogeneous sand. The cone penetration test (CPT) profiles and the blow counts confirmed that the foundation systems are comparable to each other. The experimental results of the single pile system were compared with conventional design methods. Such comparison indicated that CPT-based methods and load-transfer methods are applicable at the considered model scale. The bearing capacity prediction obtained via the Case Pile Wave Analysis Program (CAPWAP) method is conservative with respect to the static capacity. A consistent setup effect can be detected by analyzing the complete dynamic loading session. The pile group efficiency for the given foundation system was found to be less than 1.0 at both very small and very large soil strains, while it equaled 1.0 at failure.

Safety assessment of nuclear fuel reprocessing plant under the free drop impact of spent fuel cask and fuel assembly part I: Large-scale model test and finite element model validation

This paper aims to evaluate the structural dynamic responses and damage/failure of the nuclear fuel reprocessing plant under the free drop impact of spent fuel cask (SFC) and fuel assembly (FA) during the on-site transportation. At the present Part I of this paper, the large-scale SFC model free drop test and the corresponding numerical simulations are performed. Firstly, a composite target which is composed of the protective structure, i.e., a thin RC plate (representing the inverted U-shaped slab in the loading shaft) and/or an autoclaved aerated concrete (AAC) blocks sacrificial layer, as well as a thick RC plate (representing the bottom slab in the loading shaft) is designed and fabricated. Then, based on the large dropping tower, the free drop test of large-scale SFC model with the mass of 3 t is carried out from the height of 7 m–11 m. It indicates that the bottom slab in the loading shaft could not resist the free drop impact of SFC. The composite protective structure can effectively reduce the damage and vibrations of the bottom slab, and the inverted U-shaped slab could relieve the damage of the AAC blocks layer dramatically. Furthermore, based on the finite element (FE) program LS-DYNA, the corresponding refined numerical simulations are performed. By comparing the experimental and numerical damage and vibration accelerations of the composite structures, the present adopted numerical algorithms, constitutive models and parameters are validated, which will be applied in the further assessment of drop impact effects of full-scale SFC and FA on prototype nuclear fuel reprocessing plant in the next Part II of this paper.

Clogging in Pervious Concrete Pavement Made with Non-conventional Aggregates: Performance Evaluation and Rehabilitation Technique

This paper presents the clogging nature of a pervious concrete pavement using non-conventional aggregates. In order to mitigate the shortage of construction materials, brick aggregate has been preferred by many researchers as an efficient alternative to the natural stone aggregates. But the application of brick aggregate in the pervious concrete pavement is a new concept and the clogging behaviour of the pervious concrete made with waste over-burnt brick is still unknown. In this study, laboratory tests are carried out through small-scale and large-scale model tests to explore the effects of various design variables in the clogging pattern. All the experimental results are validated with the statistical analysis, and a prediction model is developed to understand the clogging nature which can be further used for modelling the clogging behaviour of a pervious concrete pavement. The rehabilitation of pavement performed through pressure washing indicated a significant improvement in the permeability after de-clogging.

Research on the rainfall-induced regional slope failures along the Yangtze River of Anhui, China

During the highway construction along the Yangtze River in Anhui, China from 2015 to 2018, regional slope failures occurred frequently near the routes and constituted significant hazards to infrastructures. Especially from June to September in 2016 and 2017, the high-temperature weather and intensive rainfall hit this region, triggering a lot of slope failures. These slope failures have two puzzling features: (1) low height (2.5–5 m) or gentle dip angles (8–25°). Such height and dips are unlikely to fail in theory; (2) slope failure emerged immediately during rainfall, while the slope materials consist of clay soil with extremely low permeability. Field investigations, laboratory tests, and a large-scale slope model test were conducted to investigate the failure modes and mechanism of the slope failures. The results show (1) low steep slopes generally show failure modes of surface erosion, or repeated local failures around the slope shoulder, while the gentle slopes often display failure modes of overall failure or even landslides; (2) the slope material mainly contains clay mineral of illite and displays strong shrinkage ability, which is prone to forming desiccation cracks during drying evaporation. Desiccation cracks can significantly improve the infiltration capacity of soils with three or four orders of magnitude. Shear strength of the soil is sensitive to water and decreases sharply with the increased water content; (3) the large-scale slope model test confirms that desiccation cracks can induce slope failure by providing preferential flow pathways for rainwater to rapidly infiltrate into deep soils. Based on the above results, the difference of failure modes and scales between the steep slope and gentle slope is interpreted. It is inferred that desiccation cracks are difficult to develop stably and constantly on the inclined surface of steep slopes due to the intense surface runoff. Thus, surface erosion and shallow flow-slip dominate the failure modes of the low steep slopes. Conversely, a gentle slope surface is favorable for the development of desiccation cracks. Hence, overall slope instability or a landslide is more likely to occur in a gentle slope after long periods of drying-wetting cycles.

Development and application of large-scale model test system for underwater shield tunnels

In order to simulate the characteristics of underwater shield tunnels, a new large-scale model test system with high water pressure and high stress loading capacity was developed. The loading control mode of the hydraulic system can perform uniform loading and gradient loading. Moreover, the vertical and horizontal hydraulic loading can reach 0·8 MPa and 1·6 MPa. The water pressure loading system has a maximum capacity of 0·5 MPa. Based on an actual project, a model test of an underwater shield tunnel lining under real soil and water pressure conditions was carried out. New segment joints of the model were developed during the model testing. The lateral pressure coefficient at rest of the model strata was slightly larger than 0·5, and close to that of the field strata. It was also found that the water pressure had a reducing effect on the soil pressure under large buried depth and high water pressure conditions. This system can mostly meet the requirements of underwater or underground engineering model tests.

Large scale model testing to investigate the influence of granular cushion layer on the performance of disconnected piled raft system

This paper presents the results of 1g model tests performed on instrumented model foundations, i.e., unpiled raft, single piled raft, single disconnected piled raft (DPR) and 3 × 3 DPR in sand under vertical load by using different granular materials and thickness of the granular cushion layer to investigate the effects of granular cushion on the performance of DPR system. The Settlement Efficiency (η) of the DPR which is nonlinear in nature is found to increase as the cushion thickness decreases and the mean particle size (d50) of the cushion material increases. A cushion thickness of two times the pile diameter and cushion material having d50 more than or, equals to 2 mm might serve as the suitable cushion material for the optimum performance of 3 × 3 DPR. The neutral axis and the zone of occurrence of maximum axial load of the pile in DPR are found to move downward with increasing cushion thickness. The piles in DPR are found to carry 35% of the total superstructure load within the settlement range considered in the study.

Deformation Responses and Mechanical Mechanism of Existing Tunnel due to New Building Construction

This study mainly investigated the variation law and mechanical mechanism of surrounding earth pressure and deformation of an existing tunnel, caused by dewatering, foundation excavation, building loading, and groundwater recovery. According to different dewatering schemes, two modes were established: nonisolated dewatering unloading‐loading mode and isolated dewatering unloading‐loading mode. Through large‐scale similar materials model test, the variation law of deformation and surrounding earth pressure of adjacent tunnel under complex unloading‐loading conditions was preliminarily revealed. Based on the size of the prototype project, the test results were further verified by the numerical simulation. The mechanical mechanism of tunnel deformation in different construction stages was analyzed after the comparative study. The results show that the tunnel structure deformation, vertical and horizontal displacement, and torsion occur in the process of building construction. In the nonisolated dewatering unloading‐loading mode, the decrease or rise of the groundwater level significantly reduces (maximum 27.85%) or increases (maximum 35.19%) the surrounding earth pressure of the tunnel. The decrease or increase of the horizontal total stress was much greater than that of the vertical total stress, which leads to the deformation of tunnel structure. The vertical displacement of the tunnel is generally settlement, which mainly occurs in the stage of dewatering and building loading, accounting for 83.21%–100.00% and 25.11%–40.34% of the total settlement, respectively. In the stage of foundation pit excavation and groundwater recovery, the tunnel rises. In the horizontal direction, the tunnel moves towards the foundation pit, mainly in the excavation stage, accounting for 82.77%–86.30% of the maximum value. Due to the uneven change of displacement field and stress field of soil outside the foundation pit, the tunnel torsion occurs. In the isolated dewatering unloading‐Loading mode, the change of groundwater has little effect on the tunnel. In the stage of excavation and construction load, the variation law of tunnel surrounding earth pressure and deformation is similar in the two modes.